Nitric oxide is a small, reactive, diffusible, biological messenger and cytotoxic agent that is involved in a wide variety of physiological processes including neurotransmission, blood clotting, blood pressure and protection against tumor cells and intracellular parasites. We wish to determine the structure of iNOS, an enzyme that contains a unique cofactor assembly, catalyzes novel monooxygenase chemistry and is involved in many aspects of human biology, and will directly address specific questions regarding the function of this enzyme including why this enzyme is not immediately inhibited by its product nitric oxide.